Download Prepared by Erhan Turgut

HISTORY OF
THE EARTH
Prepared by Erhan Turgut
Teacher: Celal ARIKOĞLU
Class: 8-A
Number: 296
The earth is about 4.5 billion years old. Geologists tried to learn the
history of it and it wasn’t easy. They worked hard and after a study of
long years, they made it. It has been revealed by examining fossils and a
lot of other things. All these things, searches and studies have formed a
branch of science that is called geology.
FOSSILS
Fossils are the remains of plants and animals. They are mostly found in
sedimentary rocks. They show us the history of the earth, the old lives,
animals and plants.
Fossilization: We said that fossils are found in sedimentary rocks. And
in order for an organism to be fossilized it must have a skeleton or shell,
and must be buried soon after death in some material that will protect it
against the destructive agents of weathering and erosion. Soft parts of
animals have been preserved under extraordinary conditions. Carcasses of
the woolly mammoth and the woolly rhinoceros have been found in the
frozen tundra of Siberia, where the flesh has been preserved ever since
the animals were entombed in the permanently frozen ground. Footprints
and trails of many different kinds of animals have been found, and even
such delicate objects as insects and feathers have been preserved as
imprints. Some of the fine-grained rocks, such as the lithographic
limestone from Bavaria, have recorded impressions of the delicate wing
membranes of flying reptiles, the fleshy tentacles of ancient squids and
jellyfish.
Casts: In some exceptional cases, fossil bones and shells are found
essentially unaltered, but most of them have undergone some changes.
After the organism has been buried in some sedimentary material, waters
circulating slowly through the ground may dissolve and remove certain
portions of the hard parts, or they may remove the entire organism,
leaving only a cavity that is called a natural mold. A cast may be obtained
from this mold by filling it with some substance like plaster of Paris. Some
natural molds show details of structure that make the casts almost as
good as the original organism.
Petrifaction: Most hard parts of animals are porous, and circulating
ground waters may fill the pores with some soluble material such as lime,
iron or silica, leaving some of the original bone or shell. Such fossils are
heavier and more durable than the originals. In other cases all of the
original material of the hard parts may be dissolved by ground waters and
some mineral put in place of the original. Such replacements may be so
detailed that microscopic structures are preserved. Some of the silicified
wood from the petrified forest near Holbrook, Arizona shows this type of
preservation. The undigested contents of the intestines, called coprolites,
are often found associated with fish, amphibians and reptiles. Such
objects sometimes contain skeletal parts of the animals that were eaten.
ANIMAL FOSSILS
Animal fossils mostly reveal the species that lived in the past and some
information about their lives. The most important animal fossils are of
reptiles and mammals.
Reptiles
Fossil reptiles are first known from rocks of Pennsylvanian age. These
remains are of reptiles that had already become differentiated into distinct
groups, which is evidence of a long pre-Pennsylvanian history. The early
reptiles differed from their amphibian ancestors only in that they
possessed the ability to lay an egg with a shell or leathery covering that
could resist drying. Thus, the reptiles were not compelled to return to the
water to lay their eggs, and were free to move into favorable upland
areas. In these regions they developed along divergent lines. No class of
vertebrates has evolved to occupy more environments than the reptiles.
Mammals
We can say that the most known three types of fossil mammals are
Carnivora, elephant and camel. So I will examine the fossil mammals in
these types.
Carnivora: The Carnivora are descended from a line of the diapsid
reptiles, known as the mammal-like reptiles. The mammal-like reptiles
were diverse. Many different lines of reptiles approached mammalian
development, but only a few reached mammalian grade. The most
progressive line of mammalian development, that of the placental
mammals, appeared in the upper Triassic. They are known as the
Pantotheria. Near the close of the Mesozoic, the Carnivora branched off
from the Pantotheria. The oldest known Carnivora, the creodonts, are
found in the lower Paleocene deposits of North America. In the early
Tertiary, the Carnivora divided into many lines of development. These
lines may be traced by numerous fossil forms from the Tertiary beds. G.G.
Simpson, an American paleontologist, lists 264 genera of extinct 113
genera of Recent Carnivora. Many of the extinct genera are forms
ancestral to the living Carnivora, while the rest belong to other lines of
development that flourished in the past and became extinct before Recent
times. Some of the living Carnivora are dogs, foxes, bears, raccoons,
weasels, badgers, civets, hyenas, cats, walruses and seals.
Elephant: The early ancestors of the elephants are known from Eocene
and Oligocene deposits in Egypt. Most of these forms gave rise to varied
lines of mastodons which are first known from the Miocene of Asia. They
spread into Europe during the Pliocene. The elephants by Pleistocene time
had separated into three lines of development: Loxodonta, the Recent
African elephant which roamed Europe and Asia during part of the ice age;
Elephas, the Recent Asiatic elephant of Asia; and Mammuthus, the
Pleistocene or ice age mammoths that roamed throughout Asia, Europe,
Africa and North America. The woolly mammoth, Mammuthus primigenius,
became well adapted to the glacial climate and lived until the close of the
last glacial state.
Camel: The camel developed in North America, as did the horse. The
earliest known camel is Protylopus from the upper Eocene deposits of
North America. It was slightly smaller than the early horse,
Hyracotherium. Protylopus had four toes on the front and hind feet, the
first toe having been lost. The progressive development of the camels
during the Tertiary more or less parallels that observed in the horse. In
the camels there was a continuous increase in size, and a reduction of the
second and fifth toes, with an increase in the size of the third and forth
toes. Correlated with this development was the elongation of the skull
and, slight increase in the length of the individual neck vertebrae, as well
as the increase in the height of the tooth crown, better to fit the animal
for browsing and its semi-grazing habits.
It appears that Protylopus gave rise to Poëbrotherium, an Oligocene
camel about the size of a goat. Many divergent lines of the camels
developed during the Tertiary ended in extinction. Two lines of
development from Poëbrotherium in the Miocene of North America gave
rise to our modern Asiatic camel, Camelus, and the South American Lama.
These forms migrated from North America across land bridges which
connected North America at that time with Asia and South America.
PLANT FOSSILS
We can examine plant fossils under these titles:
Compression Fossils: Plants are preserved in several ways, depending
upon the kind of plant and the conditions prevailing at the time the
sediments were deposited. The most common form of plant fossil is the
compression in which some part of the plant, usually a leaf, seed, or
branch, becomes entombed between layers of sand or mud. Flattening
and partial disintegration follow burial, but frequently a thin film of
carbonaceous material and, sometimes, the waxy cuticle that covered the
plant surface during life, remain. Peat and coal are compressions of thick
masses of plant material. If all of the plant substance decays during
sedimentation, only an imprint of the plant part will be left in the rock.
Casts: Sometimes when a thick plant part decays, it leaves a cavity in
the rock which becomes secondarily filled. This filling produces a natural
cast. Casts are often formed of seeds, roots, tree trunks, and the pith
cavities of certain plants.
Petrifactions: Under certain circumstances, buried plant material
absorbs mineral substances from the surrounding water. These minerals
may solidify within the cells and preserve the original form of the tissues.
A plant so preserved is a petrifaction. More than twenty mineral substances are known to petrify plant tissues, but the most common ones are
silica, calcium carbonate, and iron pyrites. Many of the fossil tree trunks in
the western part of North America are petrifactions, the silica for the
petrifying process having been supplied by volcanic ash which buried the
forests millions of years ago. Less common but important sources of
petrifactions are coal balls, which are calcareous, and pyritic nodules
found in some coal seams. They often contain petrified fragments of the
plants from which the coal was made.
Methods of Study: The method used in studying plant fossils depends
upon the type of preservation. Traces of the original tissue structure
sometimes retained in compressions can be examined after the thin layer
of compressed plant substance has been removed, by dissolving the rocky
matrix with acid. Fossil spores, leaves, and wood fragments can be
isolated from lignite and bituminous coal by treating the coal with
chemicals which cause it to disintegrate. These bits of fossil tissue yield
valuable information about the plants composing the coal, and they are
also useful in stratigraphic correlation of coal seams. The cell structure in
petrifactions can be studied in thin sections and on polished surfaces that
have been lightly etched with acid. Sections are made by sawing the
petrifaction into thin slices with a diamond-charged blade and then
grinding them to the desired thinness. They can also be made by the
somewhat simpler cellulose peel method. A smooth surface is prepared on
the specimen which is then etched with acid. The acid dissolves a portion
of the matrix but leaves the plant tissue more or less intact. After rinsing
and drying, the etched surface is covered with a collodion solution which is
peeled off after it dries. This peel contains a thin layer of the specimen
which is then mounted without further treatment for microscopic
examination. Each method has its advantages and disadvantages. The
peel method is less wasteful of material but the cell structure is seldom as
distinct as in sections prepared by the other method.
Value of Study: Fossil plants have been used to some extent by
geologists in correlating rock formations, but their main value is in the
light they throw on the history of plant life. The fossil record shows that
some living plant groups are old and others are more recent
developments. It also gives a fairly accurate picture of the general
characteristics of the vegetation of the earth in different past eras. Just as
a thorough knowledge of recent history is essential in understanding the
social, economic, and political trends of the present, information on the
development of the plant kingdom is an indispensable aid in the study of
many modern botanical problems.
GEOLOGY AND THE GEOLOGIC TIME SCALE
The scientific account of the development of the earth since the earliest
events that can be recorded, almost two billion years ago. This record
traces the past changes in the distribution of land and sea, the building
and wearing away of great mountain ranges, the origin and occurrence of
economic products such as coal, oil, and iron, the development,
distribution, and wasting away of massive glaciers and ice sheets, and the
evolution of life as revealed by fossils found in rocks of different ages.
Naturally the record is incomplete. Many events of earlier geologic time
have been obscured or obliterated by later events occurring in the same
region. The record as known today has been pieced out from fragmentary
evidence gathered from every possible source and from every remote
corner of the earth.
Geologic history deals largely with events occurring on or in the earth’s
crust. Because this crust is composed of rocks, it is necessary first to learn
something about the different kinds of rocks, how they are formed, and
where the different types may be found.
Kinds of Rocks
Many years of study of the rocks forming the earth’s crust have shown
that they may be divided into three dasses: igneous, sedimentary, and
metamorphic.
Igneous: These are the rocks that have been formed by the cooling and
hardening of molten material which has come from the interior of the
earth. Sometimes this molten material breaks through the earth’s crust to
the surface, as in the case of a volcanic eruption. When this material cools
and hardens, it forms a light-colored, gray, or dark-colored, fine-grained
rock called lava. In other cases the molten material melts its way upward
into the earth’s crust but cools and hardens before reaching the surface.
After many years these rocks may be exposed at the surface by the
weathering away of the overlying rocks. These rocks may be light or darkcolored but are always coarse-grained. Granite is the most common and
best-known example.
Sedimentary: Sedimentary rocks are formed by deposition by some
transporting agent such as water, wind, or ice. In most places on the
earth’s surface the bedrock is covered with a layer of decomposed rock,
called the mantle and composed largely of sand and clay. The upper part
of the mantle when mixed with organic material is known as the soil. Much
of the material forming the mantle is carried by rivers, wind, glaciers, and
other transporting agents and is deposited in thick layers in places such as
the continental shelf, the flood plains and deltas of rivers, and in shallow
seas, gulfs, and lakes. When this material accumulates to great
thicknesses, the bottom layers become compressed and hardened into
rock. These rock layers are called beds or strata. If the material were
sand, the resulting rock would be a sandstone. If it were clay, it would
form shale. Some of the materials carried by rivers are soluble, such as
calcium, carbonate or salt. When this material is carried to a lake or to the
sea, it may come out of solution to form strata of limestone or rock salt.
Metamorphic: Metamorphic rocks were formerly either igneous or
sedimentary but have been changed by having tremendous heat and
pressure applied to them. At times, contractions of the earth’s crust cause
the rocks in certain regions to be squeezed, broken, and twisted. Often
these rocks are pushed up to form great mountain ranges. The great
pressure placed on the rocks in these areas squeezes the rock materials
closer together and gives these rocks a different appearance. In this way
limestone is changed into marble, shale into slate or into a micaceous rock
called schist, and granite into a banded rock called gneiss.
Agents Working on Rocks
All the rocks forming the earth’s crust are subjected to many alterations
by agents working on them. Some of these agents break up the rocks into
mantle and soil; others cut valleys and canyons in the rocks and carry
away the mantle and soil; and others fracture, squeeze, and elevate the
rocks in certain areas to produce mountains, plateaus, and other scenic
features.
The first group of agents is known as the agents of weathering. When
any rocks are exposed to air and water they decay and break down into
sand and clay to form the surface mantle.
The second group of agents is known as the agents of erosion. These are
the streams, underground water, glaciers, and the wind. All of these are
capable of picking up the loose mantle and depositing it somewhere else.
Of these agents the one most powerful and active is running water. The
stream systems cut canyons and valleys into the bedrock and carry the
clay and sand of the mantle down to the sea. In this way the streams are
constantly lowering the level of the land. The underground water dissolves
the soluble rocks, such as limestone, and forms great underground
caverns. The glaciers and great ice sheets scour the bedrock and remove
loose material which is deposited later in other areas. The wind, working
mainly in desert areas, moves much of the mantle by creating dust storms
and moving sand dunes. The third group of agents is known as the agents
of diastrophism. These agents cause movements of the earth’s crust that
may build mountains by squeezing the rocks into great folds or by
fracturing them, causing certain regions to be elevated far above others.
These agents change the relative position of land and sea by elevating or
depressing parts of the land. Intrusions of igneous rocks, volcanic activity,
and lava flows frequently accompany these movements of the earth’s
crust. These three groups of agents have been active ever since the earth
has been in existence; and the way they have modified the earth’s crust,
plus the evolution of life, form the many interesting chapters in the
geologic history of the earth.
Criteria of Geologic Time
In geologic history, just as in human history, it is necessary to have an
established time scale so as to arrange the events of the past in
chronologic order. In geology the time scale has been built up from the
following criteria.
The Superposition of Strata: Unless a region has been affected by
excessive earth movements, the strata in that region will be in vertical
chronological order with the oldest bed at the base and the youngest at
the top. These strata may have a characteristic lithologic (rock-type)
sequence, such as sandstone, shale, limestone, shale, sandstone. In many
places the rock sequence is not complete. In a certain region there may
be an uplift and the upper part of the rocks may be removed by
weathering and erosion. A later downwarp would cause the deposition of
younger rock materials. This leaves a missing interval in the rock
sequence in this region, because when it was elevated and eroded, rocks
were being deposited in other regions. This missing interval is called an
unconformity.
Correlation: Correlation is the process of determining the relative ages
of the rocks exposed in different areas. If, in a near-by area, a sequence
of strata shows the same physical features as a sequence in the first
region, the rocks in these two areas may be correlated as being of the
same age.
Index Fossils: The process of determining the relative ages of rocks is
greatly aided by the presence of fossils, which occur in most sedimentary
strata. A fossil is the petrified part of a former living animal or plant. The
living forms that had hard parts such as shells or bones arc most often
preserved as fossils, since these parts are not so easily destroyed as the
soft, fleshy parts. Many bones of vertebrates and many shells of mollusks,
such as clams and oysters, can be found in the sedimentary rocks,
whereas animals with only soft parts, such as jellyfish and worms, are
rarely found in fossil form.
The forms of life have been changing throughout geologic time. In the
earlier sedimentary rocks very simple types are found, different and more
complex forms appearing in the younger rocks. Each type of plant or
animal lived for only a short part of geologic time. Therefore, when the
same type of fossil is found in rocks in two different areas, it can be
proved that the rocks are of the same geologic age. It is not difficult to tell
most types of fossils apart, because of the differences in shape, structure,
and ornamentation of their shells or other hard parts.
Units of Geologic Time: In analogy with human history, the time scale
in geologic history must be divided units of different rank in descending
order.
In geologic time, the terms “era,” “period,” “epoch,” and “age” may be
said, for purposes of proportion, to correspond respectively to the century,
decade, year, and human time. This comparison is not meant to imply an
era is too years long - indeed, an era may be over 300,000,000 years long
- but merely to show the relative order of the units. For example, just as a
century is composed number of decades, so is an era composed of a
number periods. Another major difference between the two scales is that
whereas the human time scale is exact, all years being of practically the
same length, geologic time is inexact and no two epochs are of the same
length.
Era: The era, the largest subdivision in geologic time, islimited by major
changes in the earth’s crust. For instance, if physical conditions have been
relatively static for a long time and then there is a time of great crustal
unrest with building of mountain ranges, changes in elevation of the
continents, rearrangement of the distribution of land and sea, and
changing climatic conditions, the previous era has ended and a new one
has begun. These major physical changes are called revolutions. They
cause corresponding changes in the forms of life, so that many that were
typical of the earlier era would become extinct, and new forms would begin to appear as the new era progressed.
Period: The periods are limited by minor crustal disturbances and
redistributions of land and sea. Two kinds of seas affect the continents.
The first of these is the epeiric sea, which submerges part of the interior
of a continent and is connected with the open ocean by a relatively small
outlet. Hudson Bay is an excellent example of a modern epeiric sea. The
second is the marginal sea, which overlaps the continental shelves and
extends inward over the coastal plains of the continents. If the epeiric and
marginal seas have been relatively static for a long period of time and
then small crustal movements and upwarping of the continents cause
them to be drained out into the open ocean, one period has ended and
another has begun.
Epoch: The epochs are limited by still smaller physical changes in the
physical conditions of the earth’s crust, such as the temporary shifting of
epeiric and marginal seas.
Age: An age is the smallest unit of geologic time during which conditions
are stable. A unit of sedimentary rock deposited in a region during an age
is called a formation. If the formation is of one type, it is usually so
designated, e.g., Trenton limestone, Dakota sandstone, Cincinnati shale.
If it is made up of more than one type of rock materials, the term
“formation” is used, e.g., Detroit River formation. The first part of the
name of a formation is taken, in American usage, from the geographic
locality where it was first studied and described. In European usage some
formations are named the same way, but others are named from their
physical characteristics or from their most typical index fossil.
The Archeozoic Era
The oldest exposed rocks on the continents were formed during the
Archeozoic Era. This oldest era does not start with the beginning of the
earth, however, because there is an extensive lost interval between the
time the earth became a planet and the time when the oldest rocks still
preserved were formed. Archeozoic rocks are very difficult to decipher.
Their outcrops are scattered; in most places they are covered with great
thicknesses of younger rocks. Where they are exposed they have been so
intensely metamorphosed by many revolutions and disturbances that in
many cases the original character of the rocks can not be determined.
Many long periods of erosion have removed great thicknesses of these
rocks. In addition, since no fossils occur in them, correlation of the
different areas in which they are exposed is difficult or even impossible.
One interesting fact is that the earliest Archeozoic rocks appear to be
highly metamorphosed former sedimentary rocks, the older rocks on
which they were originally deposited having been remelted and destroyed
by great numbers of igneous intrusions. Hence no trace of the original
crust of the earth remains.
In North America there are three major areas where Archeozoic rocks
are exposed. The first of these is the large region of northeastern Canada
that extends around both sides of Hudson Bay. This region is known as
the Canadian Shield. Parts of this region are covered with younger rocks
but a large percentage of the area has rocks of Archeozoic age forming
the bedrock. The earliest Archeozoic rocks known in this region are a
series of marbles, slates, and schists, interbedded with altered lavas.
Originally these formations were strata of limestone, and shale on which
the lavas were placed. After the deposition of these formations,
tremendous earth movements occurred in this region and great intrusions
of igneous rock melted their way upward into these formations. The
crustal movements resulted in the intense metamorphism of the
formations. After a long period of erosion, scattered outcrops of these
highly metamorphosed rocks appeared between large areas of the
intrusive granite.
The second area of extensive Archeozoic rocks in North America is the
piedmont region of the eastern United States, between the Blue Ridge
Mountains and the Atlantic Coastal Plain including the Great Smoky
Mountains of North Carolina and Tennessee.
The third area is in the Rocky Mountains. Because of their great
elevation, the younger rocks have been removed by erosion and the
Archeozoic rocks form the crests of many of the ranges and individual
peaks, such as Pikes Peak.
In Europe the major area in which Archeozoic rocks are exposed is in the
region of the Scandinavian Peninsula, about where Norway, Sweden, and
Finland are now located. This region is called the Baltic Shield, and, like
other areas in which Archeozoic rocks form the bedrock, is composed of
granites and highly metamorphosed former sedimentary rocks. Similar
areas are present in east central Siberia, China, western Australia,
southern Africa, and northeastern South America.
The Proterozoic Era
After an extensive period of erosion, the lands were worn down and
again parts of the continents were downwarped and invaded by shallow
seas, and other low basins began to be filled with continental deposits.
These events begin the history of the Proterozoic Era. In North America
the Proterozoic rocks are exposed in four major areas. The first of these is
the southern part of the Canadian Shield, where great thicknesses of shale
and sandstone were deposited in the areas around Lake Superior and
northeast of Lake Huron. Some of these rocks are of marine origin and
others are terrestrial; this distribution shows that the positions of the
shallow seas changed a great deal during the era. These rocks are
interbedded with large lava flows in many places. After the Proterozoic
rocks were deposited, earth movements took place in the region and the
rocks were squeezed and folded to form extensive mountain ranges. Many
Proterozoic rocks are present in the piedmont region east of the
Appalachian Mountains. These were originally deposited as strata of
limestone and shale, but metamorphism produced by mountain building in
this region at the end of the era has altered the rocks to marble, slate,
and schist. In the Grand Canyon a thick series of Proterozoic sandstones,
shales, and limestones lie unconformably on top of the Archeozoic rocks,
and in the northern Rocky Mountains a series of Proterozoic limestones
approaching 15,000 ft. in thickness were deposited. These limestones
have been carved by glacial action, to form the beautiful scenery of
Glacier National Park in northwestern Montana. Although the Proterozoic
strata in these western areas have been affected by earth movements
causing them to be folded and faulted, these movements were not intense
enough to produce metamorphism and therefore these rocks retain their
original sedimentary structures.
In Europe extensive areas of Proterozoic rocks are present on the Baltic
Shield in the form of highly metamorphosed marbles and slates. In
northwest Scotland a Proterozoic sandstone over 10,000 ft. thick overlies
Archeozoic granites and schists. Large areas of Proterozoic rocks occur in
western China, central Australia, south Africa, and central South America.
In central Australia the rocks consist of great thicknesses of
unmetamorphosed sandstones and shales; in the region of eastern Brazil
and southern Venezuela they are highly metamorphosed slates and
schists.
It is in the Proterozoic rocks that the first primitive traces of former living
organisms have been found. In the metamorphosed limestones of
Proterozoic age in western North America, limestone structures built by
primitive seaweeds occur. In addition, a few fragments of primitive shelled
animals have been found, testimony that both plant and animal life were
in existence during the era. These remains are very rare, an indication
that most forms of life, in addition to being primitive in structure, had not
yet developed hard parts such as skeletons or shells capable of being
preserved as fossils.
At the end of the Proterozoic Era, extensive mountain building took place
on all the continents. The rocks were folded, fractured, and squeezed, and
all the continents elevated far above sea level. Afterward there was a
period of erosion lasting millions of years. During this time the stream
systems gradually lowered the land surfaces until much of the upland had
been eroded down to low plains. The products of erosion were carried out
into the ocean basins, forming sedimentary rocks in areas now covered by
the seas, so that there is a tremendous interval of time between the
Proterozoic and Paleozoic eras during which no record exists in the rocks
on the present continents. During this time, called the Lipalian interval,
there must have been a great impetus to the evolution of life, because
many highly developed forms of invertebrate animals were in existence at
the beginning of the Paleozoic Era.
The Paleozoic Era
After the lands had been brought to a low level by the extensive
erosional interval at the end of the Proterozoic, certain parts of the
continents were down-warped below sea level and were occupied by
shallow seas. Throughout the Paleozoic Era minor changes in elevation of
the continents and local mountain building caused these seas to change in
size and to shift from one position to another. However, certain areas
were consistently low and under water over long periods of time. Some of
these had a linear, troughlike appearance and are called geosynclines.
They were bordered on one side by an elevated area. Continuous erosion
of this elevated area furnished sediments which were carried by stream
action into the geosynclines. The weight of these sediments caused the
geosynclines to be further depressed, leaving space for more sediments.
In this way as much as 40,000 ft. of Paleozoic sedimentary rocks accumulated in these troughs. In North America two large geosyndines developed
at the beginning of the Paleozoic Era. One of these, called the Appalachian
geosyndline, extended from the North Atlantic Ocean across southeastern
Canada and southward to the Gulf of Mexico along the axis of the presentday Appalachian Mountains. The other geosyncline extended from the
Arctic Ocean just east of Alaska southward through eastern British
Columbia and western Alberta, and through eastern Nevada and western
Utah, finally emptying into the Pacific Ocean across southern California.
These geosynclines divided North America roughly into three parts. At
different times during the era the central part was partly submerged and
the two geosynclines were connected by shallow seas. At other times
continental upwarps caused the seas to retreat from the geosynclines, and
materials eroded from the neighboring uplands were deposited there.
Similar physical conditions existed in the other continents during the
Paleozoic Era. In Europe, at different periods of the era, extensive seas
covered Norway, the British Isles, parts of Germany, France, Belgium, and
Spain, and a tremendous area in Russia extending from the Baltic Sea
eastward to the Ural Mountains. Large areas of Paleozoic rocks are
present in parts of Siberia, China, and northern India. They form the
bedrock over much of eastern Australia, northern Africa, and northern and
central South America.
The Paleozoic Era is divided into seven periods of unequal length,
separated by short periods of uplift during which no sedimentary rocks
were deposited on the continents.
Cambrian Period: The first of these periods is known as the Cambrian.
Cambria is the old Roman name for Wales, and the period received this
name because the rocks were first studied in this region. In North America
the two geosynclines were submerged during this period and in the latter
half of the period the central part of the continent was depressed so that
shallow seas connected the two troughs. Beds of sandstone, shale, and
limestone were deposited in these areas. In Europe and Asia there was a
great advance of the sea, and most of the continent was submerged with
the exception of three large and several small, isolated land masses. The
first of the large land masses is the Baltic Shield, already mentioned, the
second was in the region now forming Arabia, and the third was the
southern part of India. In addition, several smaller land masses were
present in southern Europe and southern Asia. Smaller invasions occurred
in Australia and central South America. No extensive mountain building
occurred during the Cambrian.
This period preserves the first extensive record of farmer life. Although
the land masses were bare, no land plants or animals having yet evolved,
the shallow epeiric and geosynclinal seas teemed with great numbers of
invertebrate animals and marine plants. The most unusual and interesting
of the animals were the trilobites, the three-lobed ancestors of the
modern crustaceans. These bizarre animals were widespread in the
Cambrian seas, and their horny skeletons are found in rocks of this age in
every continent. The trilobites formed about 6o per cent of the animal life
of the Cambrian Period. In addition, there were many types of
brachiopods (lamp shells), mollusks, and other forms of invertebrate life.
In fact, all the major forms of invertebrates were present in the Cambrian
seas with the exception of the corals, the bryozoans (moss animals), and
the pelecypods (bivalve mollusks, such as the modern clams and oysters).
At the end of the Cambrian Period the lands were elevated and the seas
retreated from the continents.
Ordovician Period: The second period of the Paleozoic era is called the
Ordovician. The name comes from an ancient tribe in Wales called the
Ordovicii by the Romans. During this period the continents were again
depressed and the geosynclines and other low basins were occupied by
shallow seas. As much as 70 per cent of North America was submerged
during this period and many thick beds of limestone and shale were
deposited. Much of Europe and Asia was covered by the sea at this time.
Parts of Australia and central South America were inundated, but Africa
seems to have remained above water.
All the forms of invertebrate animals that had appeared in the Cambrian
Period had representatives in the Ordovictan.
In addition, the first corals, pelecypods, and bryozoa appeared at this
time. The Ordovician Period is notable as being the time during which the
first vertebrate animals appeared. The remains found consist of skeletal
parts of primitive fish and were discovered in a sandstone of Ordcvkian
age in Colorado. These fish were jawless and the front parts of their
bodies were covered with bony plates that were fused to form a protective
armor. These fish have been named the ostracoderms.
The Ordovician Period ended with continental uplifts and local mountain
building that caused the seas to retreat. In western New England the
rocks were folded and squeezed to form the Taconic Mountains, which
extended along the east side of the Hudson Valley from Connecticut to
northeastern Canada. In Wales and western England a small mountain
range was formed by folding of the Cambrian and Ordovician rocks.
Silurian Period: The Silurian Period was also first studied in Wales. The
name is derived from the Silures, the Roman name for another ancient
tribe that lived in that region.
After the uplifts terminating the Ordovician Period there was a short
period of erosion and then the continents were again depressed and the
seas re-entered the low areas. In North America the seas were narrowly
restricted in the lower, or earlier, part of the period. In the Middle Silurian,
however, further continental downwarp caused a widespread sea to cover
almost 6o per cent of the continent. A thick limestone, the Niagara
limestone, was deposited in this sea. This formation is named from
Niagara Falls, where it forms the lip of the falls. In the upper part of the
period the seas were more restricted. Thick beds of salt were deposited in
an area extending from Michigan to central New York.
In Europe and Asia the Silurian seas were widespread and covered
almost the same areas as the earlier seas of Cambrian time. The major
land masses of the Cambrian were also above the sea during the Silurian,
as were large parts of northern China and eastern Siberia. Thick
limestones were deposited in northern Europe around the south end of the
Baltic Shield. Parts of them are covered by the present Bat. tic Sea.
Smaller seas occupied parts of eastern Australia, northern Africa, and
central South America.
In general the same major types of life characteristic of the Ordovician
Period are found in the Silurian rocks. No land plants have appeared as
yet. Among the types of invertebrate animals the corals became far more
abundant and formed massive coral reefs in the limestones in many
places. The trilobites, so characteristic of the Cambrian and Ordovician
rocks, lost their dominant position and are much fewer in number and
variety in the Silurian. In the later part of the period many large
scorpionlike animals called the eurypterids appeared.
The Silurian Period closed in North America without extensive earth
movements. In western Europe, however, a large mountain range, the
Caledonian Mountains, was formed. This range extended from Norway
through Scotland into Ireland. Similar mountain building took place in
northern Siberia, causing most of that large area to be so strongly
elevated that it remained above water for the remainder of geologic time.
Devonian Period: After a short erosional interval, parts of -the
continents were again depressed, and shallow seas entered these low
areas, beginning the physical history of the Devonian Period. The rocks of
this period were first studied in England. In northern England and parts of
Scotland the sea was prevented from entering by the presence of the
newly elevated Caledonian Mountains, but the erosion of these mountains
caused thick deposits of terrestrial sandstones to be formed in flood plains
along their flanks. This formation called the Old Red sandstone, is famous
for its well-preserved. fossil fish. Southern England was covered by the
sea at this time and thick limestones were deposited in Devonshire, from
which county the period receives its name. Much of northern Europe was
inundated during this period and many beds of shale and limestone were
deposited. The Rhine has cut its valley through these strata in the Eifel
district of southwestern Germany, producing the scenic cliffs of Devonian
limestone which rise on both sides of the valley in this region.
Devonian seas covered much of Russia, southern Siberia and south
China. An extensive sea covered central and western Australia, which had
been above water since the Cambrian Period. In South America the seas
covered parts of the central and western sections of the continent and
extended in a narrow trough from east to west through the Amazon basin.
Devonian strata are very extensive in North America. The two major
geosyndlines were under water over most of the period. During the Middle
Devonian the sea extended across the Mississippi Valley and deposited
many beds of limestone.
In the Upper Devonian thick shales and sandstones were laid down in
eastern and east central North America. These coarse deposits were the
result of a period of mountain building that began in the latter part of the
Middle Devonian and continued to the end of the period. This mountain
range extended along the east side of the Appalachian geosyndlifle from
southeastern United States to southeastern Canada. -This region was
strongly elevated, squeezed and folded in the northern part, and intruded
by large masses of granite. These granites form the White Mountains of
New Hampshire, Stone Mountain in Georgia, and many other individual
mountains in this region. The original range formed in the Upper Devonian
is called the Acadian Mountains. Erosion of these mountains as they were
being elevated caused much coarse material to be deposited westward in
the region of the Appalachian geosyncline. These deposits formed beds of
sandstone which reached a thickness of over 5,000 ft. in places. They
form the bedrock in the region of the Catskill Mountains, and the
formation has been named the Catskill sandstone. Minor mountain
building also occurred in parts of western Europe at this time. The
mountain building and elevation of the continents caused a retreat of the
seas and the termination of the Devonian Period.
Several major advances in the evolution of life took place during the
Devonian Period. The first undoubtedly land plants have been found in
terrestrial strata of this age in many parts of the world. Many types of
ferns, including the giant tree ferns, 40 ft. high and 3 ft. in diameter, have
been found near Gilboa, N. Y.
Among the invertebrate animals, the sponges, corals, bryozoa,
brachiopods, and mollusks were in great abundance. A few types of
trilobites were present, although they had declined greatly in number and
variety since the Silurian. Among the vertebrate animals, the Devonian
has often been called the age of fishes because of the great evolutionary
development of this class at this time. The primitive ostracoderms still
were present, but more advanced forms became predominant. Sharklike
fish reached a length of 20 ft. The lungfish also appeared at this time.
These fish had modified the swimming bladder into primitive lungs so that
they were able to live for a time out of water, and some of them had
developed flipperlike paired fins. In the Upper Devonian we find the first
trace of land animals. These were large, salamanderlike amphibians called
the Stegocephalia. Their skeletal structures show that they had evolved
from the lungfish by a further advance in the structure of the lungs and by
the modification of the flipperlike fins into limbs.
Mississippian Period: After a relatively short interval the continents
again were depressed and their lower parts covered by shallow seas; so
began the Mississippian Period, which derives its name from the thick
limestones deposited in the Mississippi Valley at this time.
In Europe, much of England, Belgium, and northern France was
submerged during the entire period, and thick beds of limestone were laid
down. Parts of southern Europe and southern Asia were submerged and
thick strata of shale and sandstone were deposited. Some of these beds
are of continental origin and contain many land plants and some coal
beds. The fact that Mississippian formations are not widespread in Africa
and Australia indicates that these continents were largely above water
during this period, as was South America. Small areas of Mississippian
limestones have been found in west central Argentina and north central
Colombia.
In North America the north entrance to the Appalachian geosyncline had
been closed by the building of the Acadian Mountains, and the
Mississippian Sea entered the south end of the geosyncline and the
Mississippi Valley by way of the Gulf of Mexico. Smaller seas occupied
portions of the western part of the continent. The sea covered most of the
Mississippi Valley and deposited many beds of limestone and shale. One of
the limestone strata, the Indiana limestone, is a well-known building stone
and has been used in constructing many of the government buildings in
Washington, D.C.
At the close of the period extensive mountain building occurred in
Europe. Several ranges were formed, extending from southern Ireland
across southern England, northern France, and into southern Germany.
These ranges have been named the Vaniscan Mountains. In North America
local uplifts occurred in the southern part of the Mississippi Valley. These
earth movements caused a withdrawal of the seas from the continents,
ending the period.
In general, the life of the Mississippian Period is similar to that of the
Devonian. In addition to more types of tree ferns, the plant life of the
Mississippian includes the scale trees and the first scouring rush trees
(calamites). The forms of invertebrate life were quite similar to those of
the Devonian with the exception that the cninoids (sea lilies) were far
more common in the Mississippian. Among the fossil vertebrates the
sharklike fish were numerous, and the Stegocephalia were present in
increasing numbers.
Pennsylvanian Period: As the Pennsylvanian Period began, conditions
were changing rapidly on the continental masses. The seas became more
restricted, so that continental deposits are far more widespread. In
Europe, the northwestern part of the continent was above water over
most of the period. A broad epeiric sea, the Uralian Sea, was present over
a large area in northern and central Russia, and a prom. inent geosyncline
extended across southern Europe and southern Asia, following the trend of
the present-day Alps, Caucasus, and Himalaya Mountains. This trough has
been named the Tethys geosyncline or Tethys Sea. It remained in this
general region for many succeeding geologic periods. The remainder of
Asia remained above water during most of the period.
In England, Belgium, and Germany the land was near sea level. Because
of many small, oscillating changes in altitude an alternation of marine and
continental conditions resulted in this district. When above sea level, the
region was low and swampy, so that the growth of great forests of tree
ferns, scale trees, and scouring rush trees was encouraged. Advancing
seas would bury these forests under layers of sediment and the woody
tissues would become consolidated into peat and finally into coal. Small
seas covered pants of eastern Australia at this time. In South America a
sea overlapping from the west covered large parts of Bolivia and Peru. The
south central part of Africa became depressed at this time and formed a
low, landlocked basin which received continental sandy beds of
tremendous thickness. The basal part of these -strata, named the Karroo
beds, is of Pennsylvanian age.
At the beginning of the Pennsylvanian, in North America, -the
Appalachian geosyncline was closed at both ends and terrestrial
sandstones were deposited over the cistern and central United States. In
the middle and later parts of the period, the interior of the continent was
low as in western Europe, and alternating periods of shallow sea invasions
and swampy lowlands caused the accumulation of thick beds of peat;
these have been converted into the large coal fields that extend from
Pennsylvania to eastern Kansas. Parts of western North America were
under water during much of the period, and beds of limestone, shale, and
sandstone were deposited. Local mountain-building occurred in central
United States and parts of Europe and southern Asia during the
Pennsylvanian Period, but there was no widespread deformation.
The widespread continental conditions at this time provided a great
impetus to the evolution of land plants and animals. Tremendous forests
of tree ferns and scale trees covered the extensive swampy lowlands.
Great numbers of insects and spiders lived in these forests. One species of
insect, the largest known in geologic history, looked much like the modern
dragon fly but had a wing spread of 29 in. The Stegocephalia became far
more diversified, and some of them reached a length of over 10 ft. In
North America alone, over go species of these giant salamanderlike
creatures have been found in the swampy deposits of the Pennsylvanian
period. The earliest known reptiles have been found in rocks of this age,
but their remains are too fragmentary to give much information regarding
their appearance and characteristics. Apparently they were primitive
alligatonlike forms not very different in basic structure from the
Stegocephalia.
Permian Period: The changing physical conditions of the continents
that began in the Pennsylvanian became more pronounced in the Permian
Period. This period, the last in the Paleozoic Era, was named from the
former government of Perm in eastern Russia. At the beginning of the
period the sea was present in the Ural geosyncline, a trough running
north-south along the trend of the prescnt Ural Mountains. A very shallow,
intermittent sea was present oven pants of England, northern France, and
southern Germany, in which alternate marine and continental beds of
sandstone, limestone, shale, and rock salt were deposited. The Tethys Sea
was present throughout most of the period and thick beds of limestone
were deposited in the region of northern India and the present-day
Himalaya Mountains. Very thick Permian deposits occur in eastern and
central Australia and in the islands of the East Indies. In Africa the larger
portion of the lower part of the Karnoo beds is Permian in age. Permian
strata are widely distributed in Brazil, Bolivia, and Argentina.
Many of the Permian formations in northern India, Australia, Africa, and
South America are of continental origin. In all four localities much of the
continental beds consists of consolidated glacial drift, indicating that a
major glacial period, centered mainly in the Southern Hemisphere, occurred during the Permian Period.
In North America the Penmian seas were far more restricted than were
those of previous periods of the Paleozoic Era. The major invasion
extended from the western part of the Gulf of Mexico, northward through
Mexico, and covered the south central part of the United States. The
center of this epeinic sea was in New Mexico, where the thick Capitan
limestone was deposited. This formation had been honeycombed by
underground water to form the famous Carlsbad caverns. Farther to the
east, in Kansas and Oklahoma, near-shore strata consisting of red shales
were deposited. In the upper part of the period the sea became more
restricted and thick beds of salt and gypsum were laid down. Erosion of
the gypsum beds in the Tularosa Basin of New Mexico has formed the
White Sands, and this region has been made a national monument.
Toward the end of the period, widespread mountain building began.
Contractions of the earth’s crust folded and squeezed the thick
sedimentary rocks that had been placed in the Appalachian geosyncline
during the Paleozoic Era, forming the Appalachian Mountains.
The plant life of the Permian Period was similar to that of the
Pennsylvanian with the exception that the individual plants were smaller
and not so numerous, indicating that the climate of the Permian was
cooler and drier. Invertebrate animals were quite similar to those of the
Pennsylvanian. The great evolutionary advance took place among the
vertebrate animals. In every continent the terrestrial beds of Penmian age
contain numerous remains of reptiles, some of which grew to a length of
over g ft. These ancestors of the Mesozoic dinosaurs were still primitive in
structure and had a lizardlike or alligatorlike appearance, but some of
them developed unusual features. The dimetrodon, for example, had a
tall, saillike fin extending from neck to tail along his back. The
Stegocephalia were still numerous and some of them attained a length of
15 ft.
The extensive mountain building and upwarping of the continents at the
end of the Permian caused such great environmental changes that many
forms of life that were characteristic of the Paleozoic Era became extinct
at this time. The Permian Period is the last appearance of many of the
invertebrate forms, especially the trilobites. With changing conditions new
forms evolved to take their places.
The Mesozoic Era
The Mesozoic Era is divided into three periods. The types of rocks laid
down during this era are essentially similar to those of the Paleozoic with
the exception that continental deposits are more numerous. The forms of
life found in the Mesozoic rocks are quite different from those found in the
Paleozoic. The land plants, many groups of invertebrate animals, and
especially the vertebrate animals display many remarkable changes and
adaptations.
Triassic Period: The first period of the Mesozoic Era is called the
Tniassic Period. The name is derived from the rock formations in northern
Germany, where they show a distinct threefold character: red sandstones
at the base, limestones above them, and another series of red sandstones
and shales at the top. Large areas of Europe and Asia were covered by
epeinic and geosynclinal seas during the Tniassic Period. An epeinic sea
was present in western Europe, with its shore line where England now
stands. In this sea the typical threefold formations were deposited. The
bottom and top sandstone members are partly continental in origin. An.
other sea invaded northern Russia and extended southward into the Ural
Trough. The great Tethys Sea occupied about the same area as it did
during the Pennsylvanian and Permian periods. In this sea the thick
dolomitic limestones that now form the Dolomite Alps of northern Italy
were deposited. In south central Africa, much of the upper part of the
thick, terrestrial Karroo beds is of Triassic age. These strata are known for
the great numbers of remains of reptiles found in them. In the upper part
of the period extensive continental silts and sands were deposited in
Colombia, Venezuela, and Argentina. The reptiles found in these beds are
remarkably similar to those found in the Karroo formation.
Tniassic rocks are not so widespread in North America as they are in
Europe and Asia. The erosion of the newly elevated Appalachian
Mountains caused the deposition of red terrestrial sands and clays in
downwarped basins to the eastward. These beds are interbedded with lava
flows and sills and have been downfaulted and tilted. They now form the
bedrock in the Newark Basin of New Jersey and in the Connecticut Valley.
The strata have been named the Newark series. Shallow seas occupied
parts of western North America, where they deposited beds of limestone
and shale. Terrestrial sandstones and shales are present in the region
around the Grand Canyon of Arizona.
The life of the Triassic Period shows a pronounced change from that of
the preceding Permian. The large conifer trees had become abundant, and
many of their limbs and trunks have been found in Tniassic terrestrial
strata. In northern Arizona many silicified logs are preserved in the Chinle
shale. Weathering of the shale around them has left them exposed at the
surface to form the Petrified Forest. The cycads, plants having a slender or
rounded trunk with palm-like leaves extending from the top, also were
numerous at this time. A few forms of these plants still exist in tropical
regions. Among the forms of invertebrate animals the mollusks were by
far the most abundant in the Tniassic seas. Of these, the ammonites,
distantly related to the modern chambered nautilus, were the most
common. Many types of bivalved Mollusca were present. The greatest
evolutionary advance during the period was in the realm of the vertebrate
animals. The Stegocephalia were still common, but they were not so
numerous as the reptiles, which developed great numbers of unusual
forms. The phytosaurs were a group of Tniassic reptiles which had bodies
shaped much like those of modern crocodiles but had very narrow,
elongate jaws with sharp, conical teeth. The first true dinosaurs appeared
during this period. These reptiles showed a decided advance over their
more primitive ancestors. The legs were directed downward instead of
laterally so that instead of the sprawling gait of the crocodilelike forms,
they could walk in a mammallike fashion with the body held above the
ground. Most of the Tniassic dinosaurs developed the ability to walk on
the hind legs, balancing themselves with a long tail in the fashion of the
kangaroo. Most of the Triassic species were small, ranging from 1 to 8 ft.
in length. Several types of reptiles became adapted to Living in the sea at
this time. Among these the ichthyosaurs assumed a sharklike body and
modified the legs into finlike flippers. Another group, the plesiosaurs,
developed a flattened body and an elongated neck. The limbs were
modified into paddles. Both of these groups became more abundant later
in the Mesozoic Era.
Jurassic Period: The Jurassic Period is named from the Jura Mountains
of northwestern Switzerland, where many thick strata of limestone, shale,
and sandstone are present. One of the largest invasions of the sea took
place in western Europe at this time. A great epeinic sea covered most or
England, France, Germany, and parts of western Russia. The Tethys Sea
extended from the Atlantic across southern Spain, eastward along the
trend of the Mediterranean, and across southern Asia, emptying into the
south Pacific in the vicinity of the East Indies. Much of northern Asia was
above water during the period, although epeinic seas invaded Siberia from
the north. Continental deposits of Jurassic age are known in southern
Siberia and northern China.
In Germany, the upper Jurassic deposits contain many lagoonal, finegrained limestones from which many unusual fossils have been collected.
The famous locality at Solenhofen, Bavaria, has produced remains of
winged reptiles and the only two known specimens of the first type of
bird.
Small epeinic seas covered limited parts of Australia, especially along the
western margin of the continent. Some terrestrial deposits have been
found in the interior. Most of Africa was above water at this time, with the
exception of the northern margin, which was occupied by the southern
part of the Tethys Sea. A long, narrow geosynclinal sea extended along
the western margin of South America approximately in the position where
the Andes Mountains now stand.
In North America the Jurassic seas were very restricted, occurring only
in the western part of the continent. Thick continental deposits were laid
down in the- region of the Colorado Plateau, especially north and east of
the Grand Canyon. These consist of thick sandstones and overlying shales.
The sandstones were former dune sands laid down in a desert basin.
Erosion has carved these strata into unusual shapes to produce such
scenic features as the pinnacles of Zion National Park and the beautiful
Rainbow Bridge. The overlying shaly deposit, named the Morrison
formation, is famous for its fossil dinosaur remains. Sixty-nine species of
these giant reptiles have been found in this formation which apparently
was laid down when the region was a swampy lowland.
In general, the plant life of the Jurassic was similar to that of the
Tniassic. The cycads and conifers formed the dominant elements of the
flora. One unusual plant, the ginkgo or maidenhair tree, appeared for the
first time during this period. It had some coniferlike structures but
possessed broad leaves which were dropped each fall. The tree has been
referred to as a missing link between the conifers and the flowering
plants. The ginkgo is alive today and therefore is the oldest living type of
tree, a true living fossil.
The forms of invertebrate animals of the Jurassic Period were very
similar to Tniassic forms. Corals of the reef building type were more
numerous. Echinoids (sea urchins) and mollusks were widely distributed.
Many bivalved mollusks related to the modern oysters appeared at this
time. The ammonites were still very numerous.
Vertebrate animals were dominantly reptilian, since the Stegocephalia
had become extinct by the end of the Tniassic. The dinosaurs were at the
height of their development. Herbivorous forms such as brontosaurus and
diplodocus reverted to a four-footed means of locomotion and grew to
tremendous size. Most of them had a long neck and a long tail. They
reached a maximum length of 90 ft. and some of them weighed as much
as 40 tons. Some smaller herbivorous forms such as the stegosaurs
developed defensive armor of plates and spines. Carnivorous dinosaurs
such as allosaurus retained the bipedal habit, attained a maximum length
of 35 ft., and developed large heads with powerful jaws and sharp teeth.
Other groups of reptiles were present in great numbers. The.marine
plesiosaurs and ichthyosaurs were common in the Jurassic seas. The flying
reptiles appeared for the first time during this period. The pterosaurs, as
they are called, developed membranous, batlike wings and reduced the
weight of their bodies by developing hollow bones.
A great evolutionary advance in the Jurassic Period is the first
appearance of the birds. Two bird skeletons were found in the lagoonal
limestone of Solenhofen. They show the imprint of the feathers as well as
the bony structures. However, they were very primitive compared with
later birds. They had many reptilian characters, including sharp, conical
teeth in sockets, and long tails.
The Jurassic Period was terminated by the building of the Sierra Nevada,
a range in western North America. Toward the end of the period great
folding and intrusion of granites formed the mountain range, which
formerly extended much farther north into western Canada. The southern
part has since been re-elevated to form the present range. No extensive
mountain building occurred in the other continents at this time.
Cretaceous Period: Sedimentary rocks of the Cretaceous Period
contain many beds of soft, partly consolidated, white limestone called
chalk. Beds of this nature were first studied in the cliffs along the English
Channel in the vicinity of Dover, England, and Calais, France. The name
Cretaceous, from the Latin word creta for chalk, was applied to these
strata. The term has since been applied to deposits of this age in every
continent whether chalky or not.
Large parts of Europe and Asia were covered by the sea at this time. An
elevated land mass in central Europe caused the seas to be concentrated
into two east-west troughs. One of them extended from southeastern
England eastward across northern Germany, Poland, and western Russia,
connecting with the north-south Ural Trough at its eastern end. The other
trough, the Tethys geosyncline, followed the same course as in previous
periods across southern Europe and northern Africa. It connected with the
south end of the Ural Trough and then continued eastward across
southern Asia, emptying into the Indian Ocean in the vicinity of eastern
India and western Burma. With the exception of a few small overlaps from
the north and east, the remainder of Asia was above water during the
entire period, and continental deposits of Cretaceous age are widespread
over this great continental platform. Thick beds of chalk are common in
Cretaceous beds in western Europe. The part of the Tethys Sea which
extended across northern Africa deposited thick beds of sandstone. The
erosion of these beds has furnished much of the sand of the Sahara
Desert. Australia was also invaded by epeinic seas during the period. In
South America the Andean Trough was submerged and occupied by the
sea during most of the Cretaceous. Farther eastward, large parts of Brazil
were covered with continental silts and sands which contain many
dinosaur remains.
In North America marginal seas overlapped the Atlantic and Gulf coastal
plains, depositing sands, days, and beds of chalky limestone. Another
marginal sea overlapped the west coast, extending inward in California to
the base of the newly elevated Sierra Nevada range. However, the
greatest marine invasion is in the west central part of the continent.
During this time a great depression, the Rocky Mountain geosyncline,
developed in this region and a large sea extended from the Gulf of Mexico
across the area where the Great Plains and the Rocky Mountains now
stand northward through western Canada into the Arctic Ocean. Many
thick beds of sandstone, limestone, and shale were deposited by this sea
which represents the last large marine invasion of the continent.
The close of the period was accompanied by extensive mountain building
in South America, North America, and eastern Asia. In South America the
sediments that had been accumulating in the Andean Trough over several
geologic periods were compressed and folded to form the Andes
Mountains. The same forces acting in North America compressed the
western part of the Rocky Mountain Trough to form the Rocky Mountains.
These mountains extend across western Canada and Alaska to eastern
Asia, where they split into several ranges. Extensive volcanic activity
occurred in many places at this time. Lava flows covered all of southern
India to form the great Deccan Plateau. Smaller flows covered parts of
Arabia and eastern Africa. All the continents were considerably elevated,
so that all the geosynclinal, epeinic, and marginal seas were caused to
retreat.
Several significant advances in the forms of life took place during the
Cretaceous Period. The first flowering plants are found in rocks of this age.
The remains consist of leaves and woody tissues of trees. Many of them
are of living genera, such as the willow, oak, maple, and elm. In general
the forms of invertebrate animals were similar to those of the Jurassic
Period.
Among the vertebrate animals we have the culmination of the varied
reptilian forms. Three major groups of dinosaurs were present. The
bipedal carnivorous forms were represented by tyrannosaurus, which was
45 ft. long and stood about 18 ft. high. A group of bipedal herbivorous
dinosaurs developed during this time. They had wide, flattened jaws
shaped somewhat like a duck’s bill. Many skeletons of trachodon, the
duckbill dinosaur, have been found in the Cretaceous terrestrial rocks in
North America. A third group developed a bony shield terminating in a frill
which protected the head and neck. The genus Triceratops, a common
member of this group, had three large horns extending forward from the
protective shield.
Plesiosaurs and ichthyosaurs were present in the Cretaceous seas, and a
new group of marine reptiles, the mosasaurs, having an elongated body
and relatively small flippers, developed at this time. The pterosaurs were
highly advanced. The Cretaceous forms had lost their teeth and were
better adapted to flight than their Jurassic predecessors. One species,
Pteranodon, had a maximum wing spread of 26 ft.
Two species of birds, both retaining the reptilian character of having
conical teeth in jaw sockets, are known from the Cretaceous. One of
them, Hesperornis, the diving bird, had lost the power of flight and had
become, adapted to swimming and diving in marine waters.
The first numerous mammalian remains have been found in the
terrestrial rocks of the Upper Cretaceous. Problematical transitional forms
are known from the Triassic and Jurassic, but these may have been more
reptilian than mammalian. The Cretaceous mammals were small, primitive
forms resembling somewhat the modern shrews.
The extensive mountain building and elevation of the continents at the
end of the period caused such intense climatic and environmental changes
that many forms of life became extinct. Among the invertebrate animals
the ammonites, which had been predominant in the Mesozoic seas, all disappeared. Of the vertebrate animals, all the dinosaurs, ichthyosaurs,
plesiosaurs, mosasaurs, and pterosaurs became extinct.
The Cenozoic Era
The Cenozoic Era comprises the latest 60,000,000 years of geologic
history. The era is divided into two periods, the Tertiary and the
Quaternary, or glacial. Although the latter period is much shorter, covering about the last 1,000,000 years of geologic time, it is important for the
many changes produced by the great ice sheets and because it saw the
development of man.
Tertiary Period: Many parts of Europe, Asia, and northern Africa were
covered by epeiric and geosynclinal seas during the Tertiary Period. In the
early part of the period a sea covered southeastern England, northwestern
France, and Belgium, depositing a thick series of unconsolidated sands
and clays. The Tethys Sea was still in existence. It extended from the
Atlantic Ocean across Spain, Italy, and northern Africa, thence along its
usual track in southern Asia, through the Near East and northern India,
finally emptying into the northeastern part of the Indian Ocean. Thick
strata of limestone were deposited in the regions covered by the Tethys
Sea. One of these, the nummulitic limestone, covers much of northern
Egypt. Blocks of this formation were used in the building of the Pyramids.
A small epeiric sea covered southeastern Australia at this time, and most
of the islands of the East Indies were submerged and receiving sediment.
In South America the Tertiary seas were confined to the north and south
ends of the continent. An epeiric sea covered eastern Colombia and
northern Venezuela, and a smaller one was present over southern
Patagonia. Thick terrestrial sands and silts were deposited in the Amazon
Basin.
In North America marginal seas covered the Atlantic and Gulf coastal
plains and the west coast. Thick terrestrial sediments, produced by the
erosion of the newly elevated Rocky Mountains, were deposited over the
Great Plains and in the basins between the mountain ranges.
In the middle of the period a great amount of mountain -building began
to take place in many regions. In Europe the Alps and Caucasus Mountains
were elevated by compression oI the northern part of the Tethys Trough.
The Himalaya Mountains of southern Asia are a part of the same mountain
chain. In North America the Coast Ranges of California and Oregon and
the Cascade Mountains of Oregon and Washington were formed during the
latter part of the period.
The most significant development in the forms of Tertiary life is the
evolution of the mammals. Modern plants had already become well
established by the Cretaceous, and the majority of Tertiary invertebrate
animals were direct descendents of Cretaceous forms. Modern bony fish
became more numerous in the Tertiary, but amphibians and reptiles
decreased in numbers and varieties. The mammals had a tremendous
evolutionary advance during the period. From the simple shrewlike forms
that first appeared in the Cretaceous sprang the many diversified forms
that became well established by the early part of the Tertiary. The earliest
members of the horse and elephant families are found in rocks deposited
near the beginning of the period, and shortly afterward the carnivorous
and the cloven-hoofed animals were present. Tremendous numbers of
forms developed, many of which became extinct before the end of the
period. Some reverted to a marine existence as had certain reptiles in the
Mesozoic, resulting in the whales and porpoises with limbs modified into
flippers. The bats became adapted to flight by producing elongated fingers
separated by a membrane. By the early part of the Tertiary Period the
mammals had already taken over the domination of the lands that had
been relinquished by the dinosaurs at the end of the Mesozoic Era.
Quaternary Period: The mountain building that occurred during the
latter part of the Tertiary Period caused great elevation of the continents
and retreat of the seas. The climate became much colder, and in the
Quatemnary Period great ice sheets began to develop over northern
Europe, North America, and Siberia. In Europe the ice extended from the
Baltic Shield southward across England, northern Germany, Poland, and
western Russia. The ice sheet in Siberia was not so extensive, but still
covered many parts of northern Asia. In North America the ice sheet
covered most of Canada and extended as far southward as southern
Illinois. Except for Antarctica no ice sheets occurred in the Southern
Hemisphere. There were four major advances of the ice, between which
there were periods when it melted back. The last ice sheet disappeared
from Europe and North America between 10,000 and 15,000 years ago.
The climatic changes during the period caused accompanying changes in
the forms of life. Many of the animals of the Tertiary became extinct and
new forms, adapted to the cold, appeared. Among these were the woolly
mammoth and the woolly rhinoceros. Other animals lived farther south,
where the climate was milder. At this time the mastodon and the sabertoothed tiger roamed the southern parts of the northern continents. With
the retreat of the ice these forms became extinct, to be replaced with
modern animals. It was during the last glacial period that man first
appeared on the earth. Earlier species of man, such as Neanderthal man,
may have existed in the last interglacial period, but the modern species of
man developed during the last advance of the ice and with its retreat has
populated the earth.
GLOSSARY OF GEOLOGICAL TERMS
AGGLOMERATE: A rock composed of coarse to fine fragmental material
blown out of volcanoes.
AMYGDULE:
Mineral-filled bubble holes in lava rocks.
ANTICLINE:
An arch-shaped rock fold.
AQUIFER: A permeable, water-bearing rock layer.
ARTESIAN WELLS OR SPRINGS: Wells or springs in which hydrostatic
pressure brings water toward or to the surface.
BAR: A low, ridge-shaped deposit of sand or coarser sediment built by
water currents in rivers or along shorelines.
BARCHAN: A crescent-shaped sand dune with the horns pointed away
from the prevailing wind direction.
BASE LEVEL: The limiting level for downward erosion. For streams
flowing to the ocean it is sea level.
BATHOLITH: A very large and usually irregularly shaped igneous
intrusion.
BEDDING: The layering or stratification of sedimentary rocks.
BERGSCHRUND: The crevice between the ice and the head of the valley
of a mountain glacier.
BIOHERM: A fossil organic (coral) limestone reef.
BOMB: An ellipsoidal “blob” of lava thrown from a volcano.
CALDERA: A pit-shaped volcanic opening measuring a mile or more
across.
CIRQUE: An amphitheater-shaped basin at the head of a glaciated valley.
CONCRETIONS: Mineralized nodules found in sedimentary rocks. They
are composed commonly of such materials as silica, calcite, and iron
oxide.
CROSS-BEDDING: Individual beds or strata at an angle to the general
stratification. Also called false bedding. DEFLATION:
The blowing away of fine, loose sediment by wind.
DELTA: The accumulation of stream-borne sediment deposited at the
mouth of a stream in an ocean or lake.
DIASTROPHISM: The movements and deformation of the outer part of
the earth.
DIKE: A platehike mass of igneous rock intruded at an angle to the pineexisting rock layers.
DIP: The angle of inclination of a rock layer.
DISCONFORMITY: An eroded and sometimes irregular surface between
two sets of parallel sedimentary layers.
DREIKANTER: Sand-blasted pebbles having a three-edged or Brazil-nut
shape.
DRIFT, GLACIAL: The rock fragments—soil, gravel, and silt—carried by a
glacier. Drift includes the unassorted material known as till (ground
moraine) and deposits made by streams flowing down a glacier.
DRUMLIN: Small, oval hills of glacial till with a streamlined shape from
the movement of ice over them.
DUNE: Hill of wind-deposited sand.
EPICENTER:
Point on the surface of the earth directly above the focus
of an earthquake.
EROSION: The wearing away and transportation of materials at and near
the earth’s surface by weathering and solution, and the mechanical action
of running water, waves, moving ice, or winds which use rock fragments
as tools or abrasives.
ERRATIC: Large glacier-carried boulders foreign to the region where they
have been deposited.
ESKER: A low and frequently serpentine ridge of sand and gravel marking
the course of a stream that flowed through a channel or tunnel in glacial
ice.
EXFOLIATION: A form of rock weathering characterized by the peeling
away of conchoidal slabs from exposed rock surfaces.
FAULT: A fracture in the crust of the earth along which there has been
dislocation parallel to the fracture surface.
FAULT BLOCK: A part of the earth’s crust bounded wholly or in part by
faults.
FAULT SCARP: The cliff formed by a fault. Most fault scarps have been
modified by erosion since the faulting.
FISSURE: A crack, break, or fracture in the earth’s crust or in a mass of
rock.
FOLD: Any sort of bend or flexure in rock layers initially horizontal.
FOOTWALL:
The undersurface of an inclined fault fracture.
FOSSIL: Any recognizable organic material or structure preserved in rock.
FUMAROLE: An opening from which volcanic gases issue.
GEODE: A rock cavity partially filled with mineral crystals.
GEYSER: An erupting hot spring.
GLACIER: A body of ice which slowly spreads or moves over the land
from its place of accumulation.
GOSSAN: Weathered outcrop of an ore body. Gossans are commonly
rich in iron oxide.
GRABEN: A sunken area between two faults. HANGING WALL: The upper
surface of an inclined fault fracture.
HOMOCLINE: A rock fold in which the rock layers are all inclined in one
direction.
HOOK: A sandspit in which the seaward end is curved back sharply
toward the land.
HORST: An elevated area between two faults; the opposite of a graben.
ISOSEISMAL LINE: Line connecting points of equally felt intensity for a
particular earthquake.
JOINTS: Fractures cutting rocks in a more or less regular pattern. Unlike
fault, joints show little or no dislocation parallel to the fractures.
KAME: A hill or mound of stratified sand and gravel built by meltwater at
or near the edge of a glacier.
KETTLES: Pitlike depressions left in glacial deposits by the melting of
buried masses of ice.
LACCOLITH: A mushroom-shaped mass of igneous rock intruded into
layers of sedimentary rock. Laccoliths may be a mile or more across.
LITHOSPHERE: The rocky outer part of the earth.
LOESS: Deposit of fine wind-blown dust.
MAGMA: The high-temperature, liquid silicate solution from which igneous
rocks form by crystallization.
MESA: A flat-topped hill or mountain left isolated during the general
erosion or cutting down of a region.
METAMORPHISM: The process of alteration by rearrangement and
recrystallization under the influence of increased heat and pressure,
changing a sedimentary or igneous rock to a metamorphic rock.
MINERAL: An inorganic substance of definite chemical composition found
ready-made in nature, such as calcite or quartz.
MONOCLINE: A steplike fold in otherwise flat-lying sedimentary layers.
MORAINES: Ridges or other topographic forms built up of rock debris
deposited by glaciers. Terminal moraine ridges are built at the end or
edge of glaciers. A cover of ground moraine is left as a glacier melts away.
Lateral and medial moraines are left from material carried on the sides of
valley glaciers
NAPPE: The overriding mass along a large gently inclined thrust fault.
NORMAL FAULT: A fault along which the hanging wall has moved down
with respect to the footwall.
OUTCROP: That part of a rock formation which appears at the surface;
the appearance of a rock at the surface at the projection above the soil.
Often called an exposure.
OUTWASH: Deposits of sand and gravel washed out from the ends of
glaciers by the meltwater.
OVERTHRUST FAULT: A low-angle thrust fault.
PENEPLAIN: The nearly plane surface developed as a result of longcontinued erosion.
PHENOCRYSTS: The larger mineral grains in an igneous rock composed
of grains of two distinct sizes.
PIRACY, STREAM: The natural diversion of the headwaters of one stream
into another; usually the result of more rapid downcutting by the pirate
stream.
PLACER DEPOSIT: A mass of gravel, sand, or similar mate s, rial
resulting from the crumbling and erosion of solid rocks and containing
particles or nuggets of gold, platinum, tin, or other valuable minerals,
which have been derived from rocks or veins.
PLUG: A plug-shaped mass of igneous rock; sometimes
intruded into pine-existing rock and sometimes rising in the .~ crater of a
volcano.
PORPHYRITIC: An igneous texture which is found in rock consisting of
grains of two distinct sizes.
PYROCLASTICS: The fragmental material—blocks, cinders, and ash—
thrown out of volcanoes.
REJUVENATED REGION: Any region which has been subjected to
erosion for a greater or less length of time and re-elevated so that the
streams are renewed in activity. REJUVENATION, STREAM: An increase in the erosive of a stream due to
such causes as increased rainfall or creased slope.
ROCHES MOUTONEES: Sheepback-shaped masses of formed by glacial
erosion of rock ledges. Rounded on side from which the ice came.
SEISMOGRAPH: An instrument to record earthquake dons. The record is
a seismogram.
SIAL: The general term for the rocks predominant at surface of the earth,
rich in silicon (Si) and aluminum (Al).
SILL: A sheetlike mass of igneous rock intruded between layers of
sedimentary rock.
SIMA: The general term for the rock material to be predominant at a
depth of 10 to 20 mi. below the surface of the earth, supposedly rich in
silicon and magnesium.
SINKS: Pitlike or funnellike depressions formed by solution, or solution
and collapse, in areas of soluble rock such as limestone.
SPIT: A bar of sand or coarse sediment connected at end to land.
STALACTITE: Iciclelike calcium carbonate deposited by water dripping
from the roof of a cave.
BIBLIOGRAPHY
Geologic Time Scale
Gelişim Genel Kültür Ansiklopedisi – The cover “Dünyamız”
Gelişim Hachette
Collier’s Encyclopedia
Encyclopedia Britannica